Paper Number O44

Conceptual development: Low loss precast concrete frame building system with steel connections

2014 NZSEE Conference

P.K. Aninthaneni & R.P. Dhakal

Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand.

ABSTRACT: Monolithic reinforced concrete (RC) frames and conventional pre-cast

concrete structural systems are prone to develop severe damage under high seismic

excitations, which makes buildings using these systems less sustainable because of the

downtime and repair cost required to fully restore the functionality of the damaged

building. For this reason, researchers are exploring alternate RC building systems that

minimize the downtime and seismic losses.

In this paper, schematic development of a sustainable demountable precast RC frame

system, in which the precast members are connected with steel angles/plates, steel

tubes/plates and high strength friction grip (HSFG) bolts, is discussed. The concept of this

system allows a mechanical pin to be used in the gravity frame connections such that only

the seismic frames share the lateral force imposed by earthquakes and the gravity frames

do not damage at all in earthquakes. In the proposed precast structural system, damaged

structural elements in seismic frames can be easily replaced with new ones; thereby

rendering it a definitely repairable and low loss system, despite not being a damage

avoidance solution. The load transfer mechanism from the weak beam to the strong

column through the connection is explained and a model is proposed to analyse the

connections. Pros and cons of the proposed precast framing system and its application in

practice are also discussed in the paper.

1 INTRODUCTION

In the modern world, concrete has dominated the construction sector because of its availability and

material properties. Concrete structures are constructed in two ways: cast-in-situ and precast. Precast

concrete structures can be defined as structures where the majority of structural components are

standardized and produced in concrete yards away from the site and then transported to the site for

assembly. Precast concrete construction is being adopted in many countries for its potential

advantages. The performance of precast structural system in resisting lateral loads depends on the

behaviour of connections. The implementation of innovative ideas for connecting precast elements

together, and subsequent verification through experimental procedures, has resulted in significant

advances for the precast concrete industry in seismic regions of the world in the past two decades. For

example, in New Zealand precast concrete has been used in moment resisting frames since the 1980s

(Park 1990).

Structural behaviour of precast structures differs from monolithic cast-in-situ concrete structures.

From a general point of view, there are two alternatives to design precast structures. One choice is the

use of precast concrete elements interconnected predominantly by hinged connections, whereas the

other alternative is the emulation of monolithic RC construction. The emulation of the behaviour of

monolithic RC constructions can be obtained using either ‘‘wet’’ or ‘‘strong’’ (dry or partially dry)

connections. A ‘‘wet’’ connection between precast members uses cast-in-place concrete or grout to fill

the splicing closure. Precast structural systems with wet connections must then comply with all

requirements applicable to monolithic RC constructions. A ‘‘strong’’ connection is a connection, not

necessarily realized using cast-in situ concrete, that remains elastic while designated portions of

structural members undergo inelastic deformations under the design actions (Bournas et al 2013).

Generally “strong” dry connections are achieved with use of dowels or anchor rods, steel billets, steel

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plates, and steel angles. Many researchers have proposed dry connections with different configurations

and experimentally validated and found that these systems can be considered as semi-rigid

connections which primarily depend on dowel action for force transfer from beam to column (Elliott et

al 2003a, Mohamed 1992 & Negro et al 2012). To the author’s knowledge, there is limited research in

the development of demountable precast system with strong rigid dry connection.

The present research is focused on development of sustainable demountable precast RC frame

building system using “strong” dry connection consisting of steel angle or steel tube, stiffened steel

plates and pre-tensioned high strength frictions grip (HSFG) bolts. The main advantages of the

proposed sustainable precast frame building system are:

1. Quick to construct: Building system without use of cast-in-situ concrete, site formwork, and

can be erected in quick time.

The proposed system doesn’t need any cast-in-situ concrete; the connections between floor-

floor, floor-beam, beam-column and column-foundation are made using steel elements (i.e.

stiffened steel angle or steel tube) and pre-tensioned high strength bolts. This system can be

erected in quick time which leads to significant reduction in overhead project cost and

increased financial return due to earlier occupancy of the building.

2. Simple system: Building system is simple to analyse, design and construct.

No specialist knowledge is required in the analysis, design and construction of the proposed

precast frame building system. As the precise elements and connections to be used in the

system are simple and have been used in industry for several years, general builders can easily

erect the proposed system without much difficulty. In addition, the system does not require

very precise construction and fabrication tolerance.

3. Demountable: Building system can be demounted at any time during the life span of building.

The connections between the precast elements of the frame building are made such that the

building can be easily demounted when/if needed without damaging the components. The

proposed system enables financial savings through dismantle and reuse (rather than demolish).

4. Easily upgradable: Building system can be easily upgraded or strengthened.

The proposed building system can be upgraded if higher strength is required due to change of

building occupancy or change in design code/demand. Higher strength can be achieved by

replacing the weakest frame elements with bigger/stronger ones or by adding diagonal bracing

elements with little intervention (as the steel connection can be predesigned to accommodate

the bracing elements when/if needed in future).

5. Easy/fast to repair/Insurance compliant: System with easily replaceable damaged elements,

thereby making it an earthquake resilient building system and compliant to insurance policy

“like for like as when new”.

The damaged structural beams and columns in earthquakes can be easily replaced with new

one within short time (which leads to significantly less downtime loss); thereby rendering it a

definitely repairable and low loss system, despite not being a damage avoidance solution. The

damaged building can be recovered exactly to the original state (or stronger, if needed) in a

short time which leaves no room for ambiguity in terms of compliance to the common

insurance policy of “like for like as when new”.

This paper describes the conceptual development (including schematic layout) of the proposed

demountable precast frame building system, and available lateral load resisting options within the

proposed system. It also explores an analytical model for estimating the connection capacity.

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2 PRECAST BUILDINGS AND THEIR PERFORMANCE IN CANTERBURY

EARTHQUAKES

In New Zealand, precast reinforced concrete moment resisting frames are very common since 1980’s.

In precast building systems, joints between precast elements are normally designed to emulate

monolithic construction so that the whole structure shows equivalent monolithic behaviour during an

earthquake. There are four ways of achieving equivalent monolithic behaviour in conventional precast

frames, which are shown in detail in Figure 1.

Figure 1. Commonly used arrangements of Precast Members and Cast in Place Concrete for Constructing Moment Resisting Reinforced Concrete Frames in New Zealand. (Restrepo 1992)

In system-1, the precast beam elements are placed between columns and seated on the cover concrete

of the previously cast-in-place or precast column below and/or propped adjacent to the columns. A

precast concrete floor system is placed, seated on the top of the precast beam elements and spanning

between them. Reinforcement is then placed on the top of the beams, over the precast floor and in the

beam column joint cores. The topping slab over the floor system and the beam-column joint cores is

cast-in-situ (Fib-27 2003). In system-2, the precast beams are seated on steel shims creating a

construction joint 10 to 25mm thick. Protruding column longitudinal bars pass through precast

preformed vertical holes in the beams and protrude above the beams top surface. The holes in the

precast beam elements are formed by corrugated steel ducting. The vertical ducts and the horizontal

construction joint at the bottom of the precast beams are grouted in one operation. A precast concrete

column is then positioned above the precast beam using grouted vertical laps or grouted steel sleeves

to connect the vertical column bars (Restrepo 1992).

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In system-3, T–shaped, cruciform precast concrete elements or even multi-storey cruciform units are

used. In this arrangement the vertical column bars in the precast units are connected using grouted

steel sleeves. Cast-in-place connections of the beams for this system are identical to those employed

for System 2 (Fib-27 2003).In system-4, pre-tensioned precast concrete beam shell units are used as

permanent formwork for beams. The precast U-beams support the self-weight and construction loads

and act compositely with the reinforced concrete core when subjected to other loading in the

completed structure. Precast U-beams are generally not connected by reinforcement to the cast-in-

place concrete of the beam or column, the composite action normally comes from the bond between

the roughened inner surface of the precast U-beam and the cast-in-place concrete (Fib-27 2003).

Generally concrete moment resisting frames performed as expected in the Canterbury earthquakes.

Modern precast buildings in general did well in terms of ‘life safety’ and ‘collapse prevention’; with

the exception of two RC buildings (Uma et al 2013). In the September 2010 Darfield earthquake,

modern precast concrete buildings reportedly behaved better apart from experiencing considerable

damage to non-structural elements and contents. However, cracking in precast flooring systems due to

beam elongation, damage to staircase elements and damage in gravity load elements due to inadequate

detailing to cater for the displacement demands were observed in some modern buildings (Uma et al

2013, Kam et al 2010 & Elwood et al 2011). In the February 2011 Christchurch earthquake, the

damage to the majority of modern buildings was technically repairable, but many of these buildings

were demolished based on financial viability of the available repair options. A full compilation of

vulnerability assessment of RC buildings in general in these earthquakes has been reported (Kam et al

2011), but the authors are not aware of any report specific to the damage sustained by the precast

building stock.

3 PROPOSED DEMOUNTABLE PRECAST CONCRETE BUILDING SYSTEM

The basic objective of this system is to build a sustainable RC frame building system that can be

demounted when/if needed and easily strengthened to meet increased design demand due to change of

occupancy or change in design code, and in which damaged structural elements can be easily replaced

with new one after an earthquake. In this section, schematic layout of the overall building system,

geometric configuration of the steel connections and possible lateral load resisting options within the

proposed concept are discussed.

3.1 Schematic Layout

The schematic layout of the proposed demountable precast building system is shown in Figure 2. In

this system, precast columns with steel end plate are connected to the foundation through mechanical

fix/pin joints; precast beams are connected to the columns using steel angles or steel tubes together

with steel plates and pre-tensioned HSFG bolts, precast floor is connected to the precast beam using

bolts and steel angles, and the precast flooring elements are connected using steel plate and bolts. The

connection between different structural elements and previous research in this type of connection

systems are discussed in the following sections.

3.1.1 Floor –beam connection

Figure 3 illustrates a typical connection between precast beam and precast hollow-core floor slab using

stiffened angle and bolts. The bolt connected to floor slab is removable and the bolt connected to the

beam is embedded. The steel angle on beam side is slotted in vertical direction to accommodate

relative vertical movement between the beam and slab. Similar type of connection system is also

possible with other floor slabs like flat slab, and Tee slab. Negro et al, (2012) investigated structural

capacity of non-slotted floor to beam connections using mechanical devices like dowels and couplers

and drafted design guidelines for these connections. Details of these connections and their analytical

models can be found in literature (PCI design book 2010 & CPCI guidelines 1996). However, to the

authors’ knowledge, slotted floor-beam connections have not been explored fully.

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Figure 2. Perspective view of demountable precast frame concrete structural system

Figure 3. Steel angle connection between hollow core floor slab and precast beam

3.1.2 Beam-Column connection

The efficiency of precast concrete frames in resisting gravity and lateral loads relies on the behaviour

of beam column connections. Beam-column connections should be designed to transfer all forces.

Some connection configurations using mechanical devices are able to transfer only shear and axial

forces, these connection systems are called shear connections. Examples of shear connections are

corbel connection, and steel insert connection. Experimental results and analytical models of these

shear connection systems can be found in literature (Fib-43 2008 & Elliott 1996). There is limited

research in development of semi- rigid or rigid connections using mechanical devices. Elliott et al,

(2003a) investigated four types of semi rigid beam-to-column connectors; namely welded plate

connection, steel billet connection, single cleat connection, and double cleat connection. It was found

that although the capacity and stiffness of these connections vary significantly, they can be treated as

semi-rigid connection in analysis and design without inducing too much error. Full details of such

semi-rigid connections using mechanical connectors can be found in literature (Fib-43 2008). Figures

4 & 5 shows the proposed “strong” dry beam-column connection using stiffened angles or steel tube

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and HSFG bolts. The bolts are pre-tensioned so that the initial flexural strength depends on the

frictional resistance developed between steel surface and concrete surface. Such a connection offers

high flexural strength and rotational stiffness to ensure the connection remains in elastic state while

the beams (weakest element) reach their capacity. Figure 6 shows a typical mechanical pin connection

between a gravity load resisting beam and column, which ensures that the lateral loads are shared only

among the seismic frames. This enables the precast frame building system to be built as designed so

that no surprising damage is incurred in the gravity frame connection in future earthquakes.

Figure 4. Beam-column connection system using steel angle, steel plate, and HSFG bolts

Figure 5. Beam-column connection system using steel tube, and HSFG bolts

Figure 6. Gravity load resisting beam connected to precast column with use of mechanical pin

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3.1.3 Column-column and Column-foundation connections

Figure 7 shows a typical column-column connection and a fixed column-foundation connection which

uses steel end plate and HSFG bolts. This connection system is also called steel shoe splice

connection; they are capable of transferring high tensile force and bending moment, and allow

columns to be demounted and removed at any stage. Experimental results and analytical models for

this connection system can be found in literature (CPCI guidelines 1996, Fib-27 2003 & Fib-43 2008).

Pin column-foundation connection which uses a mechanical pin is also shown in Figure 7. Such pin

connections are to be used not just in gravity load resisting frames to transfer shear and axial forces,

but they can also be used at the column bases of seismic frames to avoid the otherwise-inevitable

damage during earthquakes. This spares from having to replace the columns after an earthquake which

can be extremely challenging in tall buildings.

Figure 7. Column-column connection, column-foundation connection using steel end plate and bolts

3.2 Lateral load resisting system: Available options

The proposed precast framing system can be unbraced or braced as required to resist the lateral loads.

In case of unbraced frames, lateral loads are resisted through flexural behavior of beams and columns

whereas in braced frame lateral loads are transferred to foundation through strut and tie action. The

column to foundation connection can be designed as fixed or pinned connection depending on the

requirement of strength and stiffness. The three structural frame options for resisting lateral loads are:

(i) frame with fixed base: (ii) frame with pin base, and (iii) frame with pin base, shear only beam-

column connections and diagonal steel braces.

The load path in an unbraced frame with fixed base and pin base under external lateral load is shown

in Figure 8. The fixed base frame offers high strength and stiffness compared to the pin base frame.

The capacity and stiffness of pin base frame can be considerably increased by addition of steel braces.

In a fixed base frame system, ground storey columns will be damaged along with beams in seismic

events, and they have to be replaced with new one after seismic events; whereas in a pin base

connection only beams (which are easy to replace) will be damaged. The qualitative comparison of

base shear capacity between unbraced frames with fixed and pin bases and braced frame with pin base

(which can be adopted as an option for new design or strengthening of a pin based frame) is shown in

Figure 9.

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Figure 8. Precast frame with fixed and pin base

Figure 9. Comparision of capacity with different frame boundary conditions

The third option available for resisting lateral load is to design the system as a braced frame with all

frame connections as shear only (i.e. pin) connections as shown in Figure 10. In this system, lateral

loads are resisted through strut and tie action in braces. The load path from the roof to the foundation

is shown in Figure 10 with colour coded bar. This system can be designed to achieve the strength and

stiffness of an equivalent rigid frame with fixed base. Steel braces act as fuse elements in resisting

lateral loads. The choice of designing a frame with or without bracing can be left to the designer and

the owner depending on the desired plastic hinge mechanism and availability of bays for bracing

without disrupting the planned use of the building.

: Plastic hinge

Lateral load

Bas

e sh

ear

capac

ity

Roof displacement

Frame with fixed base

Frame with pin base

Frame with pin base and brace

Capacity increase by addition of steel

brace

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Figure 10. Precast concrete frame with simple connections and Steel brace

3.3 Analytical model of the connection

Capacity design principle should be followed in designing the system; in particular to ensure the

“weak-beam strong-column stronger-connection” hierarchy. The load path from the beam to beam-

column connection depends on whether the bolts are pre-tensioned or not. It is assumed that the

connection capacity is limited by the frictional resistance and shear resistance of the bolts. Other

modes of failures like spalling of concrete, crushing of concrete near the bolts, bearing failure of bolts,

and bearing, tearing and block shear failures of the steel angle or steel tube can be avoided by proper

detailing. The HSFG bolts are pre-tensioned to develop the required clamping force at the interfaces of

the elements being joined. The frictional resistance between the concrete and steel surfaces subjected

to the clamping force opposes the tendency to slip due to externally applied load. Figure 11 shows the

load transfer from the beam to the connection through the frictional resistance. When friction type

bolts are designed not to slip under service loads, the design capacity at ultimate load may be

calculated as per bearing type connection shown in Figure 12. The bolts in bearing type connection are

subjected to shear force and the capacity of the connection is limited by the shear capacity of bolts.

The load transfer mechanism in the beam to column connection when there is no gap between the

beam end and the column face is shown in Figure 13. The compressive force from the beam end is

transferred to the column as bearing pressure. Flexural strength of this connection is calculated as the

product of the shear or frictional resistance of the bolts and the lever arm.

Lateral load : Simple connection

: BRB Steel brace

: Precast concrete member

: Plastic hinge

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Figure 11. Force transfer mechanism through beam end to column connection (Slip critical connection)

Figure 12. Force transfer mechanism through beam end to column connection (bearing type connection)

Figure 13. Force transfer mechanism through beam end to column connection (no gap between beam & column)

F=μ*P

P

P

P

P

M MR =φ*F*d d

F=μ*P

F: frictional resistance

between steel and concrete

surface µ: Coefficient of friction

between steel and concrete

surface

d: Depth of concrete beam

MR: Moment of resistance

M: External applied moment

φ :0.9 (if slip resistance is

designed at service load)

:0.8 (if slip resistance is

designed at ultimate load)

P: Pre-tension in bolt urface

V=fs*A

V=fs*A

A=Total shear area of bolts

fs=Shear strength of bolt

P

P P

P

MR = φ* V*d

V: Total shear resistance

d M

φ :0.8 (Strength reduction

factor)

𝑓𝑠 =𝑓𝑢𝑏

√3 Shear strength of bolt

C=α*f’c*a*b

V=fs*A or F=μ*P

P

P P

P

M

MR = φ* V*d’ or φ* F*d’

d

d': Distance from top edge

of beam to centroid of

compressive force

d’

C

b :Width of beam

f’c: Compressive strength of

concrete

a: Depth of compressive

stress block

α: 0.85 ( Equivalent stress

block parameter)

φ :0.85 (Strength reduction

factor)

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4 APPLICATIONS

The proposed demountable frame system can be easily implemented in low to medium rise residential,

industrial and commercial buildings. Given that majority of buildings in New Zealand are low to

medium rise, the proposed system can be adopted in majority of RC frame buildings to be built in

future in New Zealand. In addition, because it is easy to demount, the proposed system is perfect for

temporary structures like sports complex, parking buildings and storage houses.

Gravity-only frames or secondary elements constructed with current practice have been observed in

recent earthquakes to have either participated as part of the lateral load resisting system or deformed in

a way similar to (and along with) the main seismic frames. Consequently, damage was inevitable in

gravity frames in contrast to the intention of the designer. The proposed system allows gravity

resisting frames to be built with mechanical pin connections to the lateral load resisting system. With

this practice, the lateral loads will be shared only among the lateral load resisting seismic frames;

thereby enabling the building system to be built (and behave) as designed which spares gravity frames

from any earthquake damage.

5 LIMITATIONS AND CHALLENGES

The proposed system has to address the following limitations and challenges before being ready for

implementing into practice.

1. Difficulty in demounting/replacing damaged beams in upper stories of tall buildings after an

earthquake.

2. Practicality of replacing damaged column base in fixed base seismic frames after an

earthquake.

3. Lack of design guidelines for the proposed sustainable demountable precast frame building

system.

4. Challenges in extending the proposed demountable precast system concept to buildings with

RC shear walls.

Research is currently underway at University of Canterbury to find answers to these challenges and to

materialize this concept. For this purpose, experimental testing of a range of demountable sub-

assemblages and analytical modelling and investigation are being planned to enhance the

understanding of seismic performance of such system and to eventually establish design guidelines.

6 CONCLUSION

A new precast concrete frame building system is proposed which inherently offers unique advantages

such as; quick construction, simple, demountable and reusable, easily upgradable, quickly repairable

to insurance policy compliant condition etc. The proposed system is sustainable and can be easily

implemented into practice in all RC frame buildings. It is particularly suitable for temporary structures

because the structure can easily be demounted at any time and the components can be reused in

another structure. In the proposed precast frame system, damaged structural elements after an

earthquake can be easily detached and replaced with a new one; thereby significantly reducing the

downtime and rendering it a definitely repairable and low loss solution.

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Elliott, K.S.1996. Multi-storey precast concrete framed structures. Blackwell Science, Oxford.

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Negro, P. & Toniolo, G. 2012. Design guidelines for connections of precast structures under seismic actions, http://elsa.jrc.ec.europa.eu/publications/LBNA25377ENN.pdf, European commission.

Park, R. 1990. Precast concrete in seismic-resisting building frames in New Zealand, Concrete International, Vol.12, No.11, pp.43-57.

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